1. Field of the Invention
The present invention relates to a metallocene catalyst for preparing vinyl aromatic polymers and a method for styrene polymerization using the same, and more particularly to a transition metal half metallocene catalyst with a noble structure for preparing syndiotactic styrene polymers having high activity, superior stereoregularity, high melting point and broad molecular weight distribution and a method for preparing styrene polymers using the same.
2. Background of the Related Art
Such syndiotactic polystyrene can be generally prepared using a metallocene catalyst composed of a Group 4 transition metal in the periodic table, such as titanium, zirconium or hafnium, and one or two cycloalkanedienyl groups. The cycloalkandienyl group includes cyclopentadienyl, indenyl, fluorenyl group and their derivatives.
For example, Ishihara et al. from Idemitus Kosan Co. has proposed that syndiotatic polysterene can be synthesized with high yield by using a catalyst system prepared by combining a titanium compound with an alkyl aluminum derivative in 1985. It was the first metalloscene catalyst for synthesizing syndiotatic polystyrene. U.S. Pat. No. 4,680,353 has disclosed a process for synthesizing syndiotactic polystyrene using a catalyst composed of a Group 4 atom as a metal center and various substituents including alkyl group and alkoxyl group in the presence of a cocatalyst such as alkylaluminum derivatives. The process disclosed in this patent, however, is disadvantageous in that it requires a complicated polymer purification process after polymerization to obtain pure styrene polymer due to the use of large amounts of the alkylaluminum derivatives for the polymerization and the catalyst used in the polymerization exhibits the low catalytic activity, for example 0.8 kg-PS(mmol-metal) (mol-styrene) or lower.
U.S. Pat. No. 5,206,197 has disclosed a process for synthesizing polystyren with a high degree of syndiotacticity using a catalyst composed of a metal selected from the group consisting of Groups 3 to 10 atoms in the periodic table, a cationic organic metal compound with or without having cyclopentadienyl group, and an anion organic compound for stabilizing the cationic organic metal compound which does not affect the polymerization activity in the presence of a small amount of alkylaluminum. This process, however, is also disadvantageous in that it requires the high mole ratio of styrene to catalyst, ranging from 3,500:1 to 500,000:1, resulting in a large amount of styrene residues left without taking part in polymerization.
U.S. Pat. No. 5,597,875 has disclosed a reactor for continuously producing syndiotactic polystyrene using a catalyst composed of a first component and a second component where the first component is a metal center selected from the group consisting of Groups 3 to 6 atoms and the second component is composed of an organic metal compound having various substituents including alkyl group and alkoxy group, and alkyl derivatives, or composed of cations from an organic metal compound and anions for stabilizing the organic metal compound.
However, as described above, most of the studies for preparation of polystyrene, which has disclosed so far, are commonly focused on pursuing diversity of a catalyst, including modifying cycloalkandienyl group bounded to a titanium atom by imparting various types of substituent groups to the cycloalkanedienyl group and substituting chloro group or methoxy group bound to the titanium atom at a different position with a different simple substituent.
For example, the inventors of the present application have recently reported a process for preparing syndiotatic polystyren having much higher activity and syndiotacticity as compared to the conventional ones, using a half metalloscene catalyst in which either chloro group or methoxy group is substituted with ethanolamine group or N-alkyldiethanolamine group having a plurality of binding sites, in the following international journals: (1) Yongjo Kim, Eunkee Hong, Min Hyung Lee, Jindong Kim, Yonggyu Han and Youngkyu Do, Organometallics 1999, 18, 36; (2) Yongjo Kim and Youngkyu Do, Macromol. Rapid Comm. 2000, 21, 1148; (3) Yongjo Kim, Yonggyu Han and Yongkyu Do, J. Organomet. Chem. 2001, 634, 19; and (4) Yongjo Kim, Yonggyu Han, Jeong-Wook Hwang, Myong Won Kim and Yongkyu Do, Organometallics 2002, 21, 1127; and (5) Yongjo Kim and Youngkyu Do, J. Organomet. Chem. 2002, 655, 186. Further, the same process was issued as Korean Patent No. 0301135 (invented by Youngjo Kim, Min Hyung Lee, Yongkyu Do, Yi-Yeol Lyu, Jin Hyung Lim and Hyun-Joon Kim). Korean Patent No. 0301135 has disclosed a catalyst composed of a metal center selected from Group 4 atoms of the periodic table, either cycloalkandienyl group or its derivative, and either triethanolamine group or N-alkylethanolamine group, and a polymerization process using the same catalyst in the presence of alkyl aluminum or one of its derivatives. Korean Patent No. 0365869 by Yongjo Kim, Minhyung Lee, Sungjin Park, Youngkyu Do, Sungwoong Yoon, Kiho Choi and Bogeun Song has disclosed a new catalyst synthesized by imparting an expensive substituent with high chirality to triethanolamine group and a process for synthesizing syndiotatic polystyrene with high syndiotacticity using the new catalyst. However, the catalysts disclosed in the two Korean Patents above are disadvantageous in that they exhibit high activity only in the presence of a large amount of alkyl aluminum oxane and the constituent imparted to the triethanolamine group or N-alkyldiethanolamine group is so expensive while they have an advantage of having steric hindrance, meaning high production cost for producing polystyren. For such reasons, it was difficult to commercialize the catalysts.
Accordingly, there is still a need for a catalyst that can be produced in low cost, easily treated and exhibit high activity and stability, needing a small amount of cocatalyst such as alkyl aluminum oxane.